This invention relates to a spherical lens provided with a rod clad of a construction enabling correction of spherical aberration.
The optical coupling lenses, Fourier transformation lenses, optical pickup lenses, image relay lenses, etc. used in optoelectronics systems for optical communication, optical data processing, optical recording and playback, etc. are required to be small in size and short in focal length for permitting miniaturization of system, to excel in lens characteristics for avoiding degradation of the overall system performance, and to be simple in construction for better adaptability to mass production. The spherical lens, which has a short focal length and a simple construction, may appear to meet all these requirements. Its use is, however, not feasible because it suffers from strong spherical aberration. Maxwell's fish-eye lens and Luneburg's lens require a very large refractive gradient. With Toraldo's modified Luneburg lens and Morgan's generalized Luneburg lens, it is possible to converge light on the surface of the lens or outside the lens without aberration even though they have a refractive index of small gradient but these lenses cannot be realized with the technology for controlling refractive index distribution available at present.
The inventors formerly proposed a spherical lens of high quality specially designed to hold down the spherical aberration to a practically tolerable extent (U.S. Ser. No. 244,915 filed Mar. 18, 1981, now U.S. Pat. No. 4,422,733). The spherical lens thus pproposed comprises a spherical or hemispherical core possessed of spherically symmetrical refractive index distribution and a clad of the shape of a spherical shell possessed of uniform refractive index and formed on the periphery of the core. The spherical lens has negative lateral aberration and the lens of the shape of a spherical shell, in contrast, has positive lateral aberration. Thus, the idea of selecting for the spherical shell a specific thickness optimum with respect to the refractive index distribution peculiar to the core and combining the core and the clad satisfying this relationship has led to the perfection of a spherical lens which has little spherical aberration. Indeed the spherical lens with the clad fulfills the requirements mentioned above. It has often been desired, however, to offer functions which are desirable from the standpoint of actual application. To be specific, with the spherical lens constructed as described above, (i) the light source and the image point are separated too much from the lens to permit intimate integration of the lens with other elements, (ii) the surface area in which the clad of a uniform thickness is required to be applied is too large to permit easy manufacture of the lens, (iii) while the lens, when used for an optical pickup, requires the light to be passed through a transparent disk about 1 mm in thickness and converged on the rear side and, therefore, necessitates provision of a certain distance to the focal point, the lens having a core radius of the level of 2 mm does not permit its focal point readily to reach the space behind the disk of a thickness of the level of 1 mm because of the thickness of the clad on the disk side and the concave lens effect of the surface of this clad, and (iv) when the lens is used as an essential element in the formation of a flexible light transmission path, the number of component lenses required increases and the loss of surface reflectance accordingly increases. The spherical lens in question, therefore, has been desired to be improved to preclude the faults mentioned above.